It is generally recognized that atmospheric pollution is a serious problem existing in the world today. It is also recognized that certain nitrogen oxides contribute significantly to this problem.
There are a least six stable oxides of nitrogen but only nitric oxide (NO) and nitrogen dioxide (NO.sub.2) are troublesome from a pollution standpoint. In common with other major air pollutants, these noxious nitrogen oxides arise chiefly from the production of energy, more specifically by the fixation of atmospheric nitrogen in high temperature regions of furnaces and internal combustion engines. Another source of these noxious nitrogen oxides are inorganic and organic nitration reactions in chemical process industries. Transportation, industry and electricity generation all contribute to the problem.
Oxides of nitrogen play a dual role in air pollution, first as a pollutant in their own right and second as the initiator of complex photochemical reactions with hydrocarbons. While photochemical smog is the most visible and immediately serious consequence of pollution by these noxious nitrogen oxides, e.g., adverse effects on human health, it is recognized that the presence of these noxious nitrogen oxides in the atmosphere will adversely affect the environment even though no photochemical smog has ever been detected in the particular environment. It is most important, therefore, that noxious nitrogen oxides be removed from exhaust gases emitted from polluting sources such as power plants, nitric acid plants, automobiles and the like.
Various methods have been proposed for removing noxious nitrogen oxides from gas streams and preventing the pollution of the atmosphere with these substances.
U.S. Pat. No. 4,220,632 describes noxious nitrogen oxides in a waste gas stream such as the stack gas from a fossil-fuel fired power generation plant or other industrial plant off gas stream which are catalytically reduced to elemental nitrogen and/or innocuous nitrogen oxides employing ammonia as a reductant in the presence of a zeolite catalyst in the hydrogen or sodium form having pore openings of about 3 to 10 Angstroms.
U.S. Pat. No. 3,895,094 discloses oxides of nitrogen which are selectively removed from air-polluting exhaust gases from nitric acid manufacturing plants by reaction with a stoichiometric amount of ammonia in the presence of an acid resistant aluminosilicate molecular sieve composition as catalyst.
U.K. Patent No. GB 2,039,863 describes a process for the reduction of nitrogen oxides with ammonia, when the nitrogen oxides to be reduced are in a gas mixture containing up to 21% by volume oxygen in which the gas stream to be treated is passed over clinoptilolite predominantly in the hydrogen form, the temperature being greater than 200.degree. C., space velocity being 4000 to 20,000 hr.sup.-1 and the molar ratio of NH.sub.3 to nitrogen oxides being in the range of 0.4 to 2.5.
U.S. Pat. No. 4,473,535 discloses a process for reducing nitric oxide with ammonia in the presence of a copper exchanged mordenite catalyst at a temperature between about 300.degree. F. and 800.degree. F.
U.S. Pat. No. 4,046,888 relates to a method for selectively reducing nitrogen oxides contained in exhaust gases by using ammonia as a reducing agent and a zeolite catalyst in which the zeolite catalyst is contacted with ammonia in an amount excessive over the stoichiometric amount necessary for reduction of nitrogen oxides in an exhaust gas to thereby activate the zeolite catalyst and then, the amount of ammonia is reduced to a minimum amount necessary for reduction of nitrogen oxides to effect the catalytic reduction.